1. Technical Field
The present invention relates to a control apparatus for an alternate-current (AC) motor that controls current supply to the AC motor by detecting a current of one phase among multiple phases by using a current sensor.
2. Related Art
From a societal need of low fuel consumption and low exhaust emission, electric cars and hybrid cars mounting an AC motor as a power source of the cars are now gaining attention. For example, in a hybrid car, a direct-current (DC) power source composed such as of a secondary battery is connected to an AC motor via a power converter which is configured such as by an inverter. A DC voltage outputted from the DC power source is converted to an AC voltage by the inverter to thereby drive the AC motor.
Such a hybrid car or an electric car is mounted with a control apparatus used for the AC motor. According to a known technique, the control apparatus includes an upper control unit and a lower control unit. The upper control unit acquires signals, such as an accelerator signal and a brake signal produced by the car's driver, and calculates a torque command value according to an operating state of the car. The lower control unit controls the driving of the AC motor on the basis of the torque command value given from the upper control unit. The upper and lower control units communicate with each other to monitor the occurrence of any abnormality in the control.
For example, Patent document JP-A-2000-023499 discloses a motor control apparatus including a motor control CPU, serving as a lower control unit, which calculates d-axis current and q-axis current (d-q axis currents). Of the d-q axis currents, the q-axis current contributing to torque is transmitted to a vehicle control CPU, serving as an upper control unit, for comparison with upper and lower limits of a torque command. As a result of the comparison, it is determined whether the direction of the output torque is correct or not to monitor whether the motor control CPU is performing the calculation correctly.
On the other hand, Patent document JP-A-2008-086139 or JP-A-2004-159391 discloses a technique related to one-phase control. In performing one-phase control, a current sensor for detecting a phase current supplied to an AC motor is provided to one phase alone. A current estimate value calculated based on a current detection value of the one phase is fed back in a lower control unit to control current supply to the AC motor. Thus, the provision of the current sensor to one phase alone can contribute to reducing the number of current sensors, reducing the size of an inverter in the vicinity of its output terminals of the three phases, and reducing the cost incurred in the control system of the AC motor.
In the abnormality monitoring of output torque performed by the apparatus of Patent document JP-A-2000-023499, the U-, V- and W-phase currents are detected from the current outputted from the inverter to the motor and inputted to a feedback conversion section. In other words, this apparatus is configured on the premise of providing a current sensor to each of the three phases. A technique of using Kirchhoff's law may be applied to this case. Specifically, detection values of the current sensors of two phases may be used as bases for the calculation of the current value of the remaining one phase using Kirchhoff's law. However, still, the apparatus is configured on the premise of performing feedback control based on the current detection values of at least two phases. Therefore, this configuration cannot be applied to a control apparatus that is configured to perform feedback control on the basis of a current detection value of one phase among three phases.
Regarding one-phase control, patent document JP-A-2008-086139 proposes a technique in which, of the three-phase current command values obtained through reverse d-q conversion of the d-axis current command and the q-axis current command (d-q axis current commands), the current command values of two phases other than the sensor phase are used as they are as the current estimate values of the two phases. Another patent document JP-A-2004-159391 proposes a technique of using a current phase angle of a sensor phase, which is obtained by adding the current command phase angles of the d-q axis current command values to the electrical angle of the sensor phase. Using the obtained current phase angle, the current values of the two phases other than the sensor phase are estimated. Thus, the d-q axis current command values are converted to angles on a coordinate and the converted angles are used as they are.
In this way, in the technique of one-phase control proposed in patent document JP-A-2008-086139 or JP-A-2004-159391, current command values are used, as they are, instead of detection values, for compensation of the information which is insufficient due to the use of the current detection value of one phase alone. In other words, since the estimate values are not basically calculated based on the detection values, it is difficult to correctly derive the actual state of a real machine. Therefore, the output torque cannot be properly monitored on the basis of such estimate values.
Further, in the apparatus disclosed in Patent document JP-A-2000-023499, only the q-axis current is transmitted from the lower control unit to the upper control unit. Accordingly, basically, the upper control unit can merely determine the direction of the output torque on the basis of the positive/negative sign of the q-axis current. In the case of an IPM (implanted permanent magnet) type AC motor in particular, the information on the q-axis current alone does not enable correct estimation of the output torque for the determination of abnormality, except for the abnormality that the value of the output torque is deviated to a great extent from command torque.